In a particle accelerator with a periodic structure beam space charge force may excite resonant beam emittance growth if the particle's transverse phase advance approaches 90 degrees . A recent simulation study with the PARMILA code [D. Jeon, Phys. Rev. ST Accel. Beams 12, 054204 (2009)]10.1103/PhysRevSTAB.12.054204 has shown the feasibility of measuring the stop band of this fourth order resonance in the GSI Universal Linear Accelerator UNILAC and proposed its experimental verification, which is reported here. Measurements of transverse phase space distributions behind a periodically focusing structure reveal a fourfold symmetry characteristic of fourth order resonances as well as a resonance stop band above sigma_{0}=90 degrees per focusing cell. These experimental findings agree with results from three different beam dynamics simulation codes, i.e., DYNAMION, PARMILA, and TRACEWIN.

The new FAIR facility at GSI will include a research program with antiproton beams besides a largely extended activity with heavy ion beams. The requested antiproton production rate of 7•10 10 has to be achieved with a linac-synchrotron SIS12-SIS100 accelerator configuration. Therefore GSI needs now a high intensity, pulsed proton linac as an independent new linac, additionally to the heavy ion linac injector UNILAC. A novel compact p linac for an operating frequency of 352 MHz has been designed. It is based on an RFQ of the 4-rod type or alternatively of the 4window type followed by a CrossBar H-type (CH)-DTL ranging from 3 MeV up to 70 MeV. At an aperture diameter of 20 mm these structures will reach effective shunt impedances between 100 MΩ/m and 40 MΩ/m (high energy end). 11 independently phased cavities with focusing quadrupole triplets in the intertank sections will provide 67 MV effective voltage gain within a total length of 21 m. Beam dynamics studies are promising. Depending on the beam current, the needed transverse emittance is defined by the multiturn injection scheme into the horizontal phase space of SIS12. It is aimed to fill up to 7•10 12 protons into SIS12 within 25µs at linac currents around 70 mA and at a normalized horizontal beam emittance of 3 µm. The energy spread has to be kept within ± 1•10-3. This design was also guided by available 352 MHz klystrons with peak rf power levels slightly above 1 MW.

Transverse emittance growth along the Alvarez drift tube linac (DTL) section is a major concern with respect to the preservation of beam quality of high current beams at the GSI UNILAC. In order to define measures to reduce this growth, appropriate tools to simulate the beam dynamics are indispensable. This paper is about the benchmarking of three beam dynamics simulation codes, i.e. DYNAMION, PARMILA, and PARTRAN against systematic measurements of beam emittances for different transverse phase advances along the DTL. Special emphasis is put on the modeling of the initial distribution for the simulations. The concept of rms equivalence is expanded from full intensity to fractions of less than 100% of the beam. The experimental setup, data reduction, preparation of the simulations, and the evaluation of the simulations are described. In the experiments and in the simulations, a minimum of the rms-emittance growth was observed at zero current phase advances of about 60. In general, good agreement was found between simulations and experiment for the mean values of horizontal and vertical emittances at the DTL exit.

The crossbar H-mode (CH) cavity is an accelerating structure operated in the H 21ð0Þ mode. The robustness of the crossbar geometry allows one to realize room temperature as well as superconducting linac cavities. The shunt impedance characteristics of this structure are attractive to develop proton and heavy ion linacs in the low and medium beta range. A first room temperature eight-cell prototype has proven the feasibility of the crossbar design in terms of mechanical construction, copper plating, and cooling. An innovative rf coupling concept has been developed where two CH cavities are connected by a two gap E 010 -mode resonator which, at the same time, provides transverse focusing by a quadrupole triplet. The concept has been applied in the design of the new FAIR proton linac and a scaled model of the second cavity of this injector has been built and tested too. The full scale prototype is now under construction at the University of Frankfurt. In this paper, the room temperature CH cavity development as well as the general layout of the FAIR proton injector (70 MeV, 325 MHz, 70 mA) is presented and discussed.

In high intensity linacs emittance exchange driven by space charge coupling may lead to the well-known "equipartitioning" phenomenon if the stop band at sigma(parallel) = sigma(perpendicular) is crossed at sufficiently slow rate. This Letter is the first experimental evidence of this phenomenon in a high intensity linear accelerator, here the UNILAC at GSI. Measurements of emittances at the entrance and exit of one drift tube linac tank comprising 15 lattice cells are taken for a set of transverse and longitudinal tunes. The onset of exchange on the stop band of previously derived "stability charts" confirms theoretical predictions. The measured transverse emittance growth also compares well with results from the beam dynamics simulation codes DYNAMION and TRACEWIN.